Why, when you flip a coin over after looking at the heads side, is the picture on the tails side upside down?

All U.S. coinage is produced with what is commonly called a "coin turn."
That means that the reverse side (tails) of the coins is upside down to the obverse side (heads).
While we have researched the history of this practice, we have been unable to determine the exact reason for this custom.
The Mint still produces U.S. coinage in this manner for traditional reasons and not due to any legal requirement.